The test of wearing and tearing is of vital importance in the respect of preventive maintenance and operation life. The problem is rather complex and complicated as one has to conclude on the properties of the given equipment from the results of type tests, and also the characteristic values of the type in question should be specified by using the operation data of several single devices. This problem can be practically traced back to a single root: the used systems are open and connected to their environment through a number of elements, as well as they can not be considered as closed systems even for the duration of measurement. They form an open system from an energetic point of view (energy exchange with the environment, having the characteristic values of energy input (feeding) and energy take out (useful effect), non-deprivable substantial characteristics) because of the interactions indispensable for the operation (on which the effect of the system is directed, retroactive effects) and owing to the influences of environment (environmental loads, e.g. temperature, contamination, pressure, rain etc.), as well as on account of the user's habits and conditions (e.g. early morning usage, usual usage order, effects of usual intensity, direction etc.). Relying upon these characteristics the measured values are to be handled according to the rules of fuzzy logic, and the multitude of interactions might make impossible the parametric distribution hypotheses (e.g. Kaplan-Meier non-(semi)-parametric lifetime evaluation).
Dynamical effects and changes are noise-free only in the case of very simple and reversible cases (for energetically closed systems). This is practically a theoretical idealization, because in reality the noise is always present as the fluctuation of the given signal (measured, set, used etc.) (Robinson F N H: Noise and Fluctuations, Clarendon Press, Oxford, 1974; and Freeman J J: Principles of Noise, John Wiley & Sons, Inc. 1958).
The noise/fluctuation source is composed of versatile interactions, continuous energy and entropy/information exchange of open dynamical systems and mutual dependence of individual subsystems, and the actual noise spectrum is formed in a synergetic way (Reif F: Statistical and Thermal Physics, McGraw Hill, New York, 1965). Consequently, the desired effect is accompanied in every real case by the noise/fluctuation spectrum composed of the specific features of the dynamical system. Thus, the noise/fluctuation is a form of appearance of parameters, processes, dynamical behaviour etc. always arising, but not directly involved in a given examination. Furthermore, the noise/fluctuations provide information on the (internal and/or external) interactions of the system under study.
In the course of usual wearing tests and quality examinations, each element of the system is examined separately by using several sensors, and during the measurement one tries to eliminate or minimize the noise. Consequently, the aim at these measurement procedures is to filter the noise and create the best possible signal-to-noise ratio in order to get the most exact information possible regarding the given subsystem.
There are two fundamental strategies for the elimination of noises:                All the possible interactions are fixed and the dynamics of their changes is restricted as much as possible and handled merely as a static condition (filtering with fixed parameter).        The dynamical interaction is accepted as the source of noise, however, it is separated from the “useful” signal to be examined by applying filtering mechanisms (lock-in type filtering).        
In the case of open, dissipative systems (basically every occurrence realizing not spontaneous thermodynamical changes, e.g. heat engines, biological systems, electromagnetic radiators etc.) the reduction of noise is impossible by fixing the interactions, because the open, dissipative feature assumes the definite interaction with the environment. For this reason, at the real, irreversible dynamical systems only the second possibility could be considered, namely, noise has to be taken into account and—at the most—the dynamical methods applied may suppress it and bring out the “useful” signal as far as possible.
It has been realised that certain parts of the noise spectrum can carry information relevant for monitoring the condition of a system and this information is best obtained from the Fourier transform of a measured signal of the system. However, known methods only allow for an indirect detection of very special faults or failures of the monitored system. For example, U.S. Pat. No. 5,888,374 relates to an apparatus and method for monitoring localised pitting corrosion in metal pipes or storage vessels. Here, electrochemical probes are used for sensing electrochemical noise voltage values and electrochemical noise current values at various locations within the medium contained by the metal pipes or storage vessels under study in the vicinity of their walls. To predict the extent and rate of pitting corrosion, the data obtained in this manner are subjected to electrochemical noise analysis, wherein the high frequency part of the noise spectrum is simply screened out by calculating the root-mean-square values of the measured electrochemical voltage and current noise data prior to applying the Fourier transform. This is done as pitting corrosion is characterised by very low frequencies and high frequency noise is attributed to general corrosion. As a result of this “averaging”, however, a part of the interactions and their global effects (i.e. general corrosion) on the system under study is also screened out. This means that the result obtained as the slope of the power spectral density versus frequency is a characteristic measure of not the system as a whole but only of a subsystem with reduced (i.e. at least partially screened out) interactions.
A further important characteristics of the method and apparatus disclosed in U.S. Pat. No. 5,888,374 is that the actual measuring takes place with the probes (made of the same material as the pipes and of a further non-corroding material, for reference) and not directly with the pipes and storage vessels, that is with the system of interest. Or alternatively, the monitoring process of U.S. Pat. No. 5,888,374 is an indirect process, wherein certain conclusions are drawn with respect to the corrosion condition of the pipes and storage vessels from data measured actually with the electrochemical probes (and not with the pipes and storage vessels themselves) arranged in the vicinity of the pipes and storage vessels.